Encyclopedia of Nanoscience and Nanotechnology, Ring Structures from Nanoparticles and Other Nanoscale Building Blocks, Zhen Liu, M.Dekker

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Ring Structures from Nanoparticles and Other Nanoscale Building Blocks

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Zhen Liu Rastislav Levicky Columbia University, New York, New York, U.S.A.

INTRODUCTION Over the last two decades nanomaterials have attracted growing interest because of their unique, potentially useful electronic, magnetic, and optical properties. For example, carbon nanotubes possess tunable electronic structure from metals to insulators. Nanoparticles can act as ‘‘artificial atoms’’ which, when assembled into mesoor macroscopic structures may lead to novel functional materials. Thus self-assembled particle arrays not only preserve the properties of individual particles but can also exhibit new behaviors due to interparticle correlations and coupling. The ability to organize nanoscale components into specific geometrical arrangements, with defined interconnections, underpins much of their promise for future applications as well as current fundamental studies. Here we focus on the organization of nanoparticles and, to a lesser extent, other nanoscopic building blocks into ringlike arrangements ranging over five decades of length scale, from macroscopic ( 1 mm) to nanoscopic ( 10 nm). In addition to the simple geometry or a ring, a variety of other arrangements have been realized. For example, following earlier reports of close-packed crystalline nanoparticle superstructures,[1–5] Korgel et al. analyzed nanoparticle self-assembly into superlattices in detail highlighting the effects of size-selection, ligand coverage, and interparticle attraction.[6] Detailed reviews on the fabrication and properties of nanoparticle superlattices are now available.[7,8] Other efforts have focused on organizing nanoparticles according to highly specific biological interactions[9–19] or using self-organized polymeric media such as block copolymers to spatially template nanoparticle synthesis and distribution.[20–28] Albeit the focus of this review restricts it from broadly considering the general theme of nanomaterial self-assembly, the above and related efforts continue to make critical advances toward functional materials and devices incorporating nanoscale components. The formation of ringlike assemblies, whether over macroscopic or nanoscale dimensions, is intriguing for reasons other than simple curiosity about their physical or chemical origin. Thus continuous rings, for example, will develop circulating currents when magnetic flux is applied Dekker Encyclopedia of Nanoscience and Nanotechnology DOI: 10.1081/E-ENN 120013846 Copyright D 2004 by Marcel Dekker, Inc. All rights reserved.

across the ring.[29] This ‘‘persistent’’ current has a periodicity as a function of the enclosed magnetic flux. Moreover, the electronic states of the ring are likewise functions of the applied field, leading to magnetically tunable optical and other properties that, in addition to being fundamentally interesting, may provide unique capabilities in applications. While a variety of routes to microscopic rings have been reported, including lithography,[30] direct chemical synthesis,[31] and diffusive coalescence on free liquid surfaces,[32] self-assembly of ring structures using wetting and capillary phenomena has perhaps been the most common. By its nature, self-assembly often provides a facile fabrication route, and generally similar methods can be effectively used to organize different particle types, making these methods fairly general. In what follows, various mechanisms that have led to the formation of ring structures will be described.

SELF-ASSEMBLY OF MACROSCOPIC RINGS Ringlike patterns formed by precipitated solute particles that are visible to the naked eye, such as coffee rings, are familiar phenomena. The formation of macroscopic ( 1 mm to  1 cm) ring patterns from nonwetting droplets placed on a solid support was investigated by Deegan et al.[33] The mechanism presented by these authors invoked pinning of the contact line of the drying droplet as the carrier solvent evaporates. In order to maintain the contact line pinned at its initial position in the face of continued evaporation, a capillary flow of solvent is engendered from the interior of the droplet to its perimeter. Dispersed solute particles are carried by this resulting outflow to the edge where they accumulate (Fig. 1a). When the droplet fully dries, a ringlike residue consisting of the accumulated particles remains. Notably, in this mechanism the ring formation is largely independent of the nature of the underlying surface, the deposited solute particles, and the carrier solvent. Maenosono and collaborators formed millimeter scale annular rings by precipitating 4–6-nm CdS and CdSe nanoparticles from a suspension droplet (Fig. 1b).[34] These authors postulated that, in addition to the pinning of the droplet’s contact line 3281

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of hole nucleation possibly induced by foreign particles.[40] In the following discussion some of these mechanisms are examined in greater detail. Thermocapillary Mechanisms

Fig. 1 (a) Motion of microspheres driven by the capillary flow of solvent from the interior to the periphery in an evaporating water droplet.[33] (b) Optical microscope photograph of a CdS ring with an outer diameter of 0.23 cm. (From Ref. [34].) (View this art in color at www.dekker.com.)

as discussed by Deegan et al., convection from the suspension droplet to its perimeter where ring assembly occurs could be driven by the capillary pressure from the meniscus curvature within the porous, drying ring of nanoparticles. In this model, the capillary pressure produces an effective pressure-driven suction of solvent into the ring. Both mechanisms are potentially operative in the formation of macroscopic rings of nanoparticles and provide useful insights into how such arrays may be assembled.

SELF-ASSEMBLY OF MICRON-SIZED RINGS A number of investigators reported micron-scale annular ring assemblies of various nanometer subunits such as nanocrystals, nanotubes, and even small organic molecules. The origins of these microscopic structures are likely very different from those of macroscopic rings. Ohara and Gelbart observed micron-sized rings of nanoparticles,[35] possibly originating from the accumulation of particles along the rims of dewetting holes formed during the evaporation of nanoparticle suspensions on solid supports.[36] In these studies, the particle suspensions wetted the underlying support. Maillard and coworkers presented that thermocapillary flows engendered within an evaporating particle suspension could also lead to microrings and even hexagonal meshes of nanocrystals.[37,38] Stowell and Korgel observed the selfassembly of nanoparticles into rings and honeycomb networks.[39] The selection of a particular geometry was attributed to the variation of the thermal susceptibility of the suspension’s surface tension because of the presence of nanoparticles, triggering a transition from hole nucleation in the evaporating film to Marangoni instabilityinduced convective flow. Microrings formed from other materials include a report by Schenning et al. of such structures generated from disk-like porphyrin molecules. These authors suspected that the rings formed as a result

As discussed by Maillard et al. and others,[37–39] convective flow driven by variations in surface tension at the free interface of a thin evaporating liquid film can engender film rupture and hence the formation of holes or dry patches around which nanoparticles deposit. The physical origins of thermocapillary flows in liquid films have been well documented (e.g., see Ref. [41] for a review). For sufficiently thin (less than about 1 mm) volatile liquid films, evaporation generates a temperature gradient between the film’s lower and upper boundaries due to the removal of latent heat, leaving the free upper surface cooler. Local fluctuations in the extent of cooling cause variations in the interfacial tension of this interface, with surface tension g typically decreasing with temperature T according to a thermal coefficient B= dg/dT. As a result, the liquid from warmer surface regions where the interfacial tension is lower is pulled along the surface to spread over the cooler regions, where it is forced to move downward into the film as it cannot accumulate without limit. Simultaneously, warm liquid is fed upward to the warmer surface regions. Viscous coupling propagates this convective flow throughout the film to establish threedimensional convective cells as illustrated in Fig. 2. As evaporation proceeds, the thinning film may eventually rupture at the accompanying depressions, exposing the nearly bare substrate to form holes.[42–44] Evaporation and/or interfacial dewetting then drive the growth of the holes. When film rupture and subsequent hole growth occur in a suspension layer of nanoparticles, the particles can collect at the expanding hole rims eventually leaving behind drying patterns in the shape of rings (Fig. 3a). Possibly, the hole rims become pinned before complete drying of the film due to a build-up of frictional

Fig. 2 Illustration of thermocapillary convective flows in volatile liquid films. The mean temperature drop across the film is DT. l is a characteristic wavelength of the induced convective cells.

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Fig. 3 (a) Microrings formed from ferrite nanocrystals precipitated out of hexane after deposition on a TEM grid under air.[37,38] (b) A microring of organically functionalized Ag nanocrystals precipitated out of hexane solution.[35] (c) Porphyrin rings generated from evaporating chloroform solutions.[40] (d) Microrings of gold nanocrystals prepared by spin coating from octane solutions on a polymer film (surface defects were indicated by red circles). (e) Nanotube rings formed after irradiation of bulk suspensions with ultrasound. (From Ref. [50].) (View this art in color at www.dekker.com.)

interaction between particles accumulated at the hole rim and the underlying surface. A dimensionless Marangoni number, Ma = (BDTh)/ (rnk), is customarily used to compare the time scale for the propagation of temperature disturbances across the thickness of the film with that needed to establish convective flow.[45,46] Temperature perturbations leading to the establishment of convective flow must be sufficiently long lived, corresponding to a critical Ma value above which convective flow becomes possible, and below which Marangoni instabilities will not appear. The critical Ma depends on the boundary conditions at the film’s interfaces.[45,46] At the most basic level, convective flow is favored by greater temperature variation across the film (DT), film thickness (h), and susceptibility of interfacial tension to variation in temperature (B), but is suppressed by increases in the liquid’s kinematic viscosity (n), density (r), or thermal diffusivity (k). Wetting Mechanisms For apolar liquids and surfaces interacting purely via longrange dispersive forces, a drop of solvent placed on a solid support is typically characterized as either ‘‘wetting,’’ meaning it thins so as to fully cover the support, or ‘‘nonwetting,’’ meaning it beads up and forms a finite contact angle with the solid support. More completely, a set of classification rules can be devised that incorporate not only long-range apolar interactions but also shortrange ‘‘contact’’ forces due to polar interactions.[47] By appropriate balancing of short polar and long-range apolar interactions, additional wetting states become possible

including coexistence of thin wetting films with macroscopic, nonwetting droplets, and of dewetting thin films even when thicker layers of the same liquid wet. In considering the formation of microring assemblies from evaporating films, the approximation of either strictly (i.e., irrespective of film thickness) wetting or nonwetting liquids is usually sufficient as the surfaces and organic solvents used have been largely apolar in nature. Ohara and Gelbart theoretically analyzed microring formation from evaporating particle suspensions when the suspension wets the surface.[35] Their analysis was motivated by accompanying experiments in which dilute nanoparticle solutions in wetting organic solvents such as hexane or toluene were spread on carbon-coated TEM grids and allowed to dry.[36] Drying patterns consisting of  1-mm-diameter rings were found (Fig. 3b). As a wetting liquid film evaporates, it will continue to thin until the film thickness decreases to nearly molecular dimensions, on the order of a few nanometers. At this stage, further thinning leads to increasing loss of attractive liquid–liquid dispersive interactions, which prefer to thicken the film. Ohara and Gelbart argue that, rather than dry uniformly to zero thickness, the film may seek to maintain a minimal thickness te to counteract the loss of favorable liquid– liquid interactions, following a previous theoretical analysis for nonvolatile liquid films by de Gennes.[48] In their model, as evaporation proceeds, the drying film will develop holes which will expand so as to maintain remnant wetted regions at the thickness te. The holes would open once the film thickness decreases sufficiently for free energy barrier to hole nucleation to become comparable to the thermal background kT, where k is the

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Boltzmann constant. For typical solvents this occurs at thicknesses of  1 nm. Once holes open and grow, nanoparticles can collect at the receding rims in a manner analogous to that described above for thermocapillary flows, leaving behind drying patterns in the shape of rings. Alternately, for sufficiently thin wetting films, holes can nucleate simply by spontaneous local evaporation of liquid molecules to form a dry patch or because of the presence of surface heterogeneities (e.g., a small nonwettable region). Schenning et al. observed micron-sized ring assemblies composed of porphyrin molecules.[40] Their experimental procedure was similar to those used to form rings of nanoparticles. A droplet of porphyrin molecules in a solvent such as chloroform was placed on a substrate and allowed to evaporate for 10 sec, followed by draining of the remaining solution with filter paper. These authors suggested that the ringlike patterns originate from the nucleation of holes engendered by possibly combined influences of 1) nonwetability of the surface by the liquid; 2) progressive thinning of the liquid due to evaporation; and 3) deposition of foreign particles (e.g., from the laboratory ambient) that lowers the free energy barrier for hole nucleation. As the holes nucleate and grow, porphyrin molecules would accumulate at the rim increasing their concentration. Eventually, the concentration will exceed solubility and a condensed porphyrin phase will precipitate out to form the ring structures (Fig. 3c). Liu and Levicky observed that spin coating of nanoparticles from organic solvents on polymer films produced micron-sized rings very similar to those reported by Ohara et al. and others.[35–40] Spin coating instead of droplet drying was used to better control deposition conditions. Samples for transmission electron microscopy (TEM) were prepared as reported previously.[28] From TEM micrographs, these authors identified that microrings formed around defects in the polymer film (Fig. 3d). Based on this evidence, ring formation was attributed to defect-induced nucleation of holes in the spin-coated particle solution followed by particle accumulation at the hole rims, leaving microring drying patterns as discussed above. The defects appear white in the images (Fig. 3d), suggesting that the polymer film is thinner (possibly punctured) at those locations. Others have similarly connected surface topography with nucleation of holes and subsequent formation of drying patterns. For example, step edge defects on highly oriented pyrolytic graphite surfaces led to the formation of ringlike patterns from drying collagen solutions.[49] Microring assembly will, in general, also depend on other factors such as solute concentration and evaporation speed. Clearly, rings cannot form if the solute (e.g., nanoparticle) concentration is too low because of the insufficiency of available material. Moreover, microring

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formation, at least in wetting systems, appears favored by faster rates of evaporation. For instance, in experiments by the present authors in which nanoparticle suspensions were spin coated from relatively low volatility, wetting solvents such as octane, microrings did not form if the deposition speed was below 100 rpm or if static evaporation was used. Presumably, under such wetting conditions, a nucleated hole can quickly seal unless solvent evaporation from the hole rim is sufficient to ensure hole growth. Other Types of Microrings Microrings of carbon nanotubes have also been reported.[50,51] Martel and coworkers observed microrings of single-walled carbon nanotubes (SWNTs) following ultrasound irradiation of nanotubes in a warm solution of sulfuric acid and hydrogen peroxide (Fig. 3e). This process shortens the SWNTs and evidently can cause them to coil into rings. The physical origin of coiling was suggested to stem from tube bending due to the nucleation of bubbles on the hydrophobic surfaces of the SWNTs during ultrasound irradiation, followed by subsequent bubble collapse. Once a tube becomes coiled into a ring, the strain energy of bending can be balanced by physical van der Waals attraction along the nanotubes. This renders the rings highly permanent, sufficiently so that they can be recovered by filtering. High yields of rings, up to 50%, were reported.

SELF-ASSEMBLY OF NANOSCALE RINGS Rings with diameters ranging from 10 to 100 nm have been made out of magnetic nanoparticles or metal nanocrystals by several groups. In contrast to micronsized rings, these structures are approximately one particle wide along their perimeter (Fig. 4). The self-assembly mechanisms, as may be expected, are distinct from those effective for forming macro- or microrings and involve magnetostatic particle interactions in the case of magnetic

Fig. 4 (a) Nanorings formed by 16-nm cobalt nanoparticles.[13] (b) Nanorings composed of 2.5-nm gold nanocrystals, formed by spin coating from octane solutions.

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nanoparticles.[52,53] The present authors have also assembled nanoscopic rings from metal nanoparticles by spin coating nanoparticle solutions on thin polymer films. Magnetostatic Mechanisms Tripp et al. synthesized weakly ferromagnetic cobalt nanoparticles ( 30 nm in diameter), possessing a 3- to 0.4-nm oxide layer.[52] The nanoparticles were dispersed in toluene or CH2Cl2 in the presence of a resorcinarene as a stabilizing surfactant. Droplets of nanoparticle solution containing surfactant were deposited on carbon-coated TEM grids and allowed to dry. If the concentration of surfactant was sufficiently high, this procedure produced a large population of nanorings with diameter in the range of 50 to 100 nm. A possible mechanism for ring formation is minimization of magnetostatic energy realized when a string of magnetic dipoles (i.e., a string of ‘‘head-to-tail’’ aligned ferromagnetic Co nanoparticles) closes, thus eliminating unpaired magnetic poles at its ends. Notably, without sufficient surfactant the Co nanoparticles aggregated into dense, continuous layers or ‘‘rafts.’’ The need for the nonvolatile resorcinarene surfactant in the assembly of rings was believed to stem from kinetic stabilization attributed to the enhancement of the viscosity of the deposited particle layer. Otherwise, at low surfactant concentrations, the nanoparticles were sufficiently mobile to rearrange into more thermodynamically stable, dense aggregates. Other aggregate structures, such as oriented strings of particles, were also identified with preference for a particular geometry somewhat adjustable by application of external magnetic fields. A previous report by Puntes et al. similarly described, among many other structures, the self-assembly of nanorings from ferromagnetic Co nanoparticles (Fig. 4a).[53] Closed motifs, such as nanorings, did not form with smaller Co nanoparticles that were superparamagnetic rather than ferromagnetic. These results indicate that the permanency of the particles’ magnetic dipole plays a crucial role in facilitating the formation of closed loops or rings. In the work of Puntes et al., the Co nanoparticles had a thin organic coating and were deposited onto carbon-coated TEM grids from evaporating suspensions. Assembly of Nanorings on Polymer Films Polymer media have been used to template spatial distributions of nanometer-sized particles in a variety of ways, including in situ synthesis of nanoparticles in ordered block copolymer matrices,[54–56] introduction of premade nanoparticles into the bulk or on the surface of such media,[25,57–61] or via additional routes exploiting polymeric micelles[62,63] or multilayered structures.[64–69] Liu and coworkers have been investigating the organiza-

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tion of nanoparticles in polymer ‘‘brushes,’’[28] which are monolayers of polymer chains tethered by one end to a planar support. The polymer brushes consist of poly(ethylenepropylene) (PEP) chains and are typically between 5 and 20 nm in thickness. The PEP chains incorporated a shorter polystyrene (PS) endblock to provide anchoring to an underlying film of PS homopolymer. Dodecanethiol-stabilized gold nanoparticles about 2.5 nm in diameter were spin coated on top of the polymer brush from octane and their drying patterns imaged by TEM. On some specimens, rings of nanoparticles 10 to 50 nm in diameter were observed in high yields (Fig. 4b). The underlying physical cause of ring assembly has not yet been identified.

CONCLUSION Fabrication of structures at submicron length scales is a difficult yet crucial capability in advancing devices and technologies that may one day incorporate nanomaterials. Prospective examples of applications include novel analytical tools, ultrasmall chemical sensors, optoelectronics, and new reagents and catalysts for use in chemistry and biology. The last few years have seen rapid developments in the self-assembly of a variety of structures from nanoparticles and other components such as nanotubes or nanorods. This brief review has specifically emphasized the methods for organizing tiny building blocks into ring-shaped assemblies, with the predominance of the responsible physical phenomena traced to wetting and hydrodynamics of thin liquid films. Even as the theme of self-assembly in nanomaterial science continues to expand, as illustrated by this report, at least for specific geometries general methods can begin to be formulated.

ACKNOWLEDGMENTS During the writing of this work, Zhen Liu was supported by the MRSEC program of the National Science Foundation (DMR-0213574) and by the Donors of The Petroleum Research Fund, administered by the American Chemical Society.

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